Well, the fact that you can do something doesn't mean you should do it.
Sorry, dad.
If anyone else still cares, steps to reproduce this issue in a fresh portable installation of MusicBee, updated to 3.6.8902 P:
Two tracks with minimum tags:

Only track number and title differ.
Adding these to MusicBee shows a single album with both tracks as expected:

In the
Tag Editor, change the second
Artist value in Track 2:

Given that users are always told MusicBee doesn't store single values, but uses the semi-colon as a delimiter, it's a bit odd that this doesn't update the second tag value - it adds a
Display Artist tag with the full artist name, including semi-colon:

Okay, at least we know that happens.
Refreshing the Albums view now will show two albums with one track in each:

This view is still to be expected though, because (a) as was mentioned, not having an explicit
Album Artist tag will default to using
Artist, which is now different between the two tracks, and (b) I'm not an idiot.

Nope.
I mean, sure, you
can use the
Tag Inspector to delete the
Display Artist tag:

But after doing so, no amount of rescanning and refreshing will get MusicBee to display the two artists as "Artist 1" and "Artist 2" again.
Somewhere, MusicBee is retaining the old
Display Artist value where it can't be seen or changed by the user.
Even removing the track from the library and re-adding it will still show "Artist 1; Artist 3".

The way I've found to fix it, is to edit the
Artist value again, putting it back to the original value:
Then deleting the now irrelevant
Display Artist tag and refreshing the view.
(Obviously, in normal use you wouldn't be editing an
Artist tag just to change it back again - but editing any tag in the
Tag Editor seems to add the
Display Artist tag, which is when the issue can arise)
[Edit]
Tested with Opus, MP3 and M4A - all exhibit the same behaviour.